In a photolithography operation, a pattern from a reticle is transferred to a photoresist on a wafer. The transferred pattern may include geometrical features, such as lines with different widths, openings of different shapes and sizes (e.g., circular, elliptical, rectangular, etc.), and the like. The dimensions of each feature may be described in terms of its critical dimension (CD). For example, referring to a circular-shaped opening, a CD can refer to a width of the opening.